1. Field of the Invention
The present invention relates to an analog-to-digital conversion circuit. More particularly, the present invention relates to an analog-to-digital conversion circuit that can increase an operation frequency of a sample holder thereof.
2. Description of Related Art
Computers have become indispensable tools in modern life, and most of the information is processed by digital systems. To ensure the digital system can process analog information (for example, temperature, humidity, and luminance, etc.), an analog-to-digital conversion circuit is applied to convert the analog information into digital information.
FIG. 1 is a functional block diagram illustrating a conventional analog-to-digital conversion circuit. Referring to FIG. 1, in the conventional analog-to-digital conversion circuit 100, to satisfy a specification of high input impedance, a source follower 110 serves as an input stage. The source follower 110 receives an input voltage Vin, wherein an input range of the input voltage Vin is, for example, 0-1.5V. Since the source follower 110 has an electrical characteristic of uplifting a voltage for a bout 1V, an output range of the source follower 110 is about 1-2.5V.
Since the output range of the source follower 110 is about 1-2.5V, a first half of a sample holder 120 has to use an electronic device with an operating voltage of 3.3V to correctly sample the voltages. The sample holder 120 outputs a sampled voltage to an analog-to-digital converter (ADC) 130. The ADC 130 converts the sampled voltage into digital information and provides it to the digital system for processing.
Compared to an electronic device with an operating voltage of 1.8V, an equivalent capacitance of the electronic device with the operating voltage of 3.3V is greater. Moreover, an electron mobility of the electronic device with the operating voltage of 3.3V is lower than that of the electronic device with the operating voltage of 1.8V. Therefore, since the first half of the sample holder 120 uses the electronic device with the operating voltage of 3.3V, an operation frequency of the sample holder 120 is limited.